Numerous recent proposals such as JLAMP (JLab AMPlifier), a 4th generation light source covering the range 10 eV-100 eV in the fundamental mode with harmonics to 1 keV, and the LHeC (Large Hadron Electron Collider) Test ERL have invoked recirculation and energy recovery as a means of cost-performance optimization for linear accelerators. Use of such systems as free-electron laser (FEL) drivers can be challenging because of the combination of the deleterious impact of coherent synchrotron radiation (CSR) on beam quality during recirculation and bunch compression, and the desirability of limiting machine size and complexity.
A conventional recirculation arc 10 based on an alternating gradient second order achromatic “FODO” transport is shown in FIG. 1. The conventional recirculation arc 10 includes a sequence of quadrupole focusing and defocusing lenses 12 interleaved with bending dipole magnets 14 with rational betatron phase advances chosen to add up to an integer full phase advance over the entire arc.
As the momentum compaction of the recirculation arc 10 is nonzero, it will lead to changes in the length of an energy-chirped beam. If the chirp is generated by acceleration on the rising side of the RF waveform, the bunch will lengthen; if the chirp is generated by acceleration on the falling side of the RF waveform, the bunch will be compressed in length. Although this latter M56>0 compression has certain advantages, if employed as a means of final bunch compression, the impact of CSR is in this apparatus is both dramatic and detrimental.
Accordingly, it would be desirable to provide a method and apparatus that for bunch length compression during recirculation while also limiting and controlling beam quality degradation due to CSR.